Electrical heating device



8, 1931- I J. KERCHER ET AL 1,335,602

ELECTRICAL HEATING DEVICE Original Filed Sept. 20. 1926 :s sheeis-shet 1Dec. 8, 1931. .1. KERCHER ETAL ELECTRICAL HEATING- DEVICE Original FiledSept. 20, 1926 4 3 Sheets-Sheet 2 HIHI .57

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Original Filed Sept. 20, 1926 3 Sheets-Sheet 3 INYEN T005 14/7 /10!" \/vHerc/7e! Patented 8, 1931 PATENT OFFICE CISOO,

NATIONAL COIPANY, OF SAN IRAN- GISCO, CALIFORNIA, A CORPORATION OFDELAWARE ELECTRICAL HEATING DEVICE '-Application filed September Q0,1926, Serial No. 186,444. Renewed llovember 1!, ma

This invention relates enerall to devices for producing a supply 0 heatrom electrical energy.

Electrical heating devices for domestic or commercial use nerall utilizea resistance conductor provi ed wit a suitable insulating mounting. Itis common practice in such heatin elements to protect the conductor by asuita le casing, such for example as a metal sheath which may dissipatethe heat by radiation, by imparting the same to convection currents ofair or by direct conduction to a heat absorbing body. For example, ithas been proposed to employ a sheet metal casing enclosin one or moreresistance wires, the wires being insulated as by means of a refractorycement. Such elements have a relatively high heat lag, and due toexpansion and contraction of the sheath, the cement or other refractoryinsulating material readily breaks down, with the result that theresistance element either fails or becomes grounded upon the sheath. Asomewhat similar heating element utilizes a sheath in the form of ametal tube which is likewise filled with a refractory material, thesheath being fr uently swaged to a reduced diameter to pa iilr therefractory material about the wire.

While elements of the latter type may have a comparatively long life,they are expensive to manufacture and are difiicult to incorporate inheating appliances.

It is an object of this invention to devise an electrical heatingelement of simple construction which will have a long operative life anda relatively low heat lag.

It is a further object of this invention to devise a heating elementwhich may be readily adapted to form a part of an electrical heatingappliance. v

It is a further object of this invention to construct a heating elementhaving a sheath of such material that it may be operated at hightemperatures without injury or deterioration.

It is another object of this invention to devise a heating applianceincorporating a heating element constructed of resilient sheet metal.

Further objects of .this invention will appear from the followingdescription in which we have set forth the preferred embodiment of ourinvention. It is to be understood that the invention is to be accorded arange of equivalents consistent with the state of the prior art.

Referring to the drawings:

Figure 1 is a cross sectional view of a heating appliance incorporatingcertain principles of this invention.

Fig. 2 is a plan view showing one half of the device illustrated in Fig.1.

Fig. 3 is a cross sectional view of an oven construction illustratinganother form of electrical heating device incorporating certainprinciples of this invention.

Fig. 4 is a cross sectional view taken along the hne.44 of Fig. 3.

Fig. 5 is a side elevational view partlyin section illsutrating one formof heating element incorporated in the heating devices shown in Figs. 1to 4 inclusive.

Fi 6 is a cross sectional view taken along the line 66 of Fig. 5.

Fig. 7 is a cross sectional view of the metal sheathing incor orated inthe heating elements shown in igs. 5 and 6.

Figs. 8,9 and 10 are views similar to Figs. 5, 6 and 7, showing amodified form of heatin element.

ig. 11 is a side elevational view shown partly in section showing oneform of heating element provided with terminal connectoi's mounted uponthe sheath.

Fig. 12 is an end view of the elements shown in Fig. 11.

Figs. 13 and 14 are views similar to Figs. 11 and 12 but showing amodified form of heating element with theterminal connectors mountedupon the sheath.

The invention involves a number of novel features one of which is aheating element which is applicable to any form of electrical applianceor other heating device. The novelty in the heating element resides inthe use of a metal sheath of chrome nickel alloy which has been found topossess properties making it especially adaptable for this urpose.Instead of constructing this sheat in the form of a rigid casing ortube,

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it is made of relatively thin sheet material which is preferablypositioned upon the insulated resistance element in such a way as topress against the same by its own inherent resilience, thus being at alltimes in close thermal contact with the same. In devising an electricalheating device to incorporate such aheating element,the elements havebeen positioned in spaced relationship and supported adjacent theirends, suitable. provision being made for electrically connectingadjacent elements. The heating device shown in Figs. 1 and 2 is of thetype commonly known as an electric grill or hot plate. it includes aplurality of heating elements which are mounted in spaced relationshipupon a suitable frame ds nated generally at 11.

Sereral possible cmbudinient ing elen'ient are shown in elusive. Forexample. in Figs. elusive which illustrate the elemei as incorporated,with the heater 1 and 2. there is provided a resistance conductor 13preferably in the form of a helical or coiled wire of suitable metalsuch as a chrome nickel alloy. This resistance wire is insulated bymeans of a plurality of refractory beads or tubes 14: which arepreferably strung upon the resistance wire in contiguous relationship soas to form an articulated resistance element. About the .insulator 1dthere is disposed a sheath 16 of relatively 'thin resilient sheet metalwhich is preferably adapted to normally resiliently press against theouter surfaces of the insulators 1 In forming this sheath the sheetmetal is first bent or rolled into the form shown in Fig. 7 and theinsulated resistance element is then inserted into the sheath to expandthe same to the form shown in Fig. 6. This expansion of the sheath is ofcourse within its elastic limit so that it always tends to resilientlypress against the insulators 14;. The edges of the metal strip are notsecured together to form a closed tube but are preferably allowed tooverlap or to form a slot 17 extending longitudinally of the element.The particular metal used to form the sheath is preferably a chromenickel alloy which we have found tohave certain peculiar propertieswhich make it especially adaptable for this use. For example, a chromenickel alloy will not loose its natural resilience when heated to even aglowing temperature and under similar conditions will not oxidize orcorrode.

Another form of element is shown in Figs. 8 to 10 inclusive in which thesheath instead of being tubular in shape is formed triangular. Thus thesheath 19 has been shown as being bent to form two inclined sideportions 20 and a top portion 21. As in case of the element previouslydescribed the sheath is bent so that when the insulators wire. The

14. are inserted they serve to expand the sheath so that it will pressagainst the insulators along three lines of contact. With'thisconstruction the top portion 21 forms a relatively large flat surfacefor transmitting heat by conduction to an absorbing body.

In Figs. 11 to 14 inclusive we have illustrated how terminal connectorsmay be mounted directly upon the ends of the metal sheath. Thus forexample, as shown in Figs. 11 and 12, the metal sheath 23 which istubular in shape has been shown as formed with a flat end portion 24which is apertured to receive a terminal connector 25. The terminal 26of the resistance wire 13 is extended into electrical contact with theterminal con--. motor 25, suitable insulating members 26 being inse'"-ed between the flattened sides of the sheath r preventing grounding ofthis 1 iinal connector is of course 1th by suitable means such as insi gushers 28. Heating elements of this kind by he made up in stand ardsizes for incorporation in various types of electrical appliances. lnthe construction Lil shown in Figs. 13 and is a heating element having atriangular shaped metal sheath 30 has been shown as provided with aflattened end portion 31 for mounting a terminal connector 25, the endportion being formed for example by overlapping the side portions 20.

Referring again to Figs. 1 and 2 for a detailed description of theheating device illus-. trated therein, the mounting for the heatingelements 10 includes a casing 33 within which is disposed a ring 34which is preferably L- shaped in cross section. The upper portion 35 ofthis L has been shown as extended into contact with and secured to theupper rim of the casing 33 so as to form a peripheral flange 36. Theother portion 37 has been shown as depending into contact with thebottom 38 of the casing so as to inclose a ring-shaped space 39 formaking necessary electrical connections between the heating elements.The ends of the element 10 are extended through apertures provided inthe ring 34, these apertures being spaced so as to arrange the elementsin spaced arallel relationship to form a planar top for supporting a.cooking utensil or other body which it isdesired to heat. For retainingthe elements Within their respective apertures the ends of each casingmay be slotted tongues 40. The slot 17 is preferably arranged at thebottom of each elements in order to prevent the entrance of foreignsubstances. The electrical connections have been indicated at 41 andpreferably serve to connect alternate elements so as to form two groupswhich may be either operated independently, in series, or in parallel.Suitable terminal connectors 42. are mounted upon the casing for makingthe necessary connections to provide bent-out ISO 2, these headers Sinceliquids may be spilled through the spaces between the heating elementthe bottom 38' of the casing is preferabl inclined downwardly toward thecenter 0 the same and is provided with a central aperture 43. The innerspace of this bottom wall may be covered bysuitable heat insulatingmaterial or may be provided with a false bottom 44 having a smooth uppersurface serving to reflect heat radiated from the heating elements.

In Figs. 3 and 4 a heating device has been shown as incorporated .withan electric oven construction, the side and rear walls of the oven beingindicated at 50 and 51 respectively. The heating device has been shownas comprising a pluralitglof heating elements 52 supported ad acent teir ends in a pair of spaced metal headers 53 and 54. The elements aresecured within apertures provided in the headers in a mannersimilar tothat described previously with respect to Figs. 1 and serving to providespaces for interconnecting the wires of the different elements. Asindicated, alternate elements are connected to form two groups ofheating elements gized separately, in series or in parallel to securedifferent degrees of heat. The header 53 is provided with a suitableconnection )lock 55 carrying the bayonet connectors 56 which are adaptedto cooperate with a suitable electrical connection block 57. Theterminal connectors 58 carried by this block are for making electricalconnections to a control switch and current supply lines. The device maybe removably supported within the oven by providing a pair of guides 59mounted for example upon the top 60 of the oven and adapted to engage apair of L- shaped side brackets 61 secured to the ends of the headers 53and 54, these brackets 61 preferably running between the ends of theheaders in order to reinforce the, same.

The efiiciency of the heating elements may be increased and all dangerof short circuiting obviated by providing the convoluted resistance wirewith a thin coatingof refractory material. such as alundum. In applyingthis coating the insulators are first strung upon the coiled wire andthen the assembly is stretched or elongated and dipped in an alundumwash. The stretching of the wire provi des spaces between the insulatorsto permit the wash to flow through the same to efi'ective- 1y cover theentire surface of the wire. When removed from the wash it is permittedto return to its original length and the excess material is allowed todrain away. A thin coatingot the refractory material remains upon thewire, thus facilitating conduction of heat to the insulators andpreventing short circuiting of adjacent convolutions.

The heating devices described above have high electrical cfliciency dueprincipally to the novel construction of the heating element.

which may be either ener-' Because of the use of a thin sheet metalsheath, the heat lag is reduced to a minimum. The heat developed by theresistance wire is uniformly conducted to the sheath as all of structedso as to utilize the metal sheath as a return conductor. Thus theresistance wire at one end of the element can be connected with theadjacent end of the sheath so that the element can be connected to theline by connections made at the other end of 'the element to theresistance wire terminal and sheath. In this case the elements must beprovided with an insulating mounting to prevent grounding upon frame ofthe device.

e claim:

1. An electrical heating element comprising a convoluted resistanceconductor, CIICU- lar refractory insulating tubes strung upon saidconductor, and a longitudinally s lit resilient metal sheath enclosingsaid tu es, said sheath being normally stressed in an expanded positionto press upon the sides of said tubes thereby forming an effective heattransferring contact between the tubes and said sheath.

2. An electrical heating element comprising a resistance conductor,refractory insulating tubes strung upon said conductor, an elongatedresilient sheet metal sheath bent to encompass said tubes, said sheathhaving its sides normally sprung outwardly by the encompassed insulatingtubes and having free longitudinal edges.

3. An electrical heating element comprising a bead insulated conductor,and a resilient sheet metal sheath of chromium nickel alloy bent toencompass said conductor, said sheath having its sides normally sprungoutwardly by the encompassed insulation of said conductor and havingfree opposed edges to permit expansion and contraction of the sheath.

4. An electrical heating element comprising an elongated resistanceconductor, round insulation surrounding said conductor, and

an elongated resilient metal sheath enclosing said conductor andinsulation, said sheath being formed of a single sheet metal strip bentabout said conductor and insulation 'to form a longitudinally splitcasing, said sheath being normally expanded within its elastic limit toress against the sides of said insulation and ibrming the sole metalsheathing material for the same.

5. An electrical heating element comprising a resistance conductor,insulation surrounding said resistance conductor, an elongated resilientsheet metal sheath initially formed to provide an enclosing space havinga resistance conductor,

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having ing a cross sectional area which is smaller than thecorrespondingarea of the insulation adapted to encompass saidinsulation, said sheath being normall under internal stress and extendedwithin its elastic limit to exertpressure upon said insulation.

6. An electrical heating element comprising a resistance conductor,refractory insulation surrounding said conductor, an elongated resilientsheet metal sheath initially formed to provide an enclosing space havinga cross sectional area which is smaller than the corresponding area ofthe insulation adapted to encompass said insulation, said sheath havingits sides normally sprung outwardly by the encompassed insulation andsaid sheath free longitudinal edges.

7 An electrical heating element comprisinsulation surrounding saidresistance conductor, an elongated resilient sheet metal sheath beinginitially formed to provide an enclosing space bent to encompass saidinsulation, said sheath having a cross section of discontinuoustriangular outline the area of which is smaller than the correspondingarea of said insulation, the discontinuityallowing expansion andcontraction of the sheath.

8. An electrical heating element comprising a resistance conductor,insulation surrounding said resistance conductor, an elongated resilientsheet metal sheath bent to encompass said insulation, said sheath beinginitially formed to provide an enclosing space having a cross section ofdiscontinuous triangular outline the area of which is smaller than thecorresponding area of said insulation, the discontinuity allowingexpansion and contraction of the sheath and said sheath being normallyexpanded within its elastic limitto exert pressure upon said insulation.

9. The method of manufacturing an electrical heating element having aninsulated resistance conductor enclosed within a metal sheath, whichmethod comprises placing the insulation about the conductor, initiallyforming the sheath so that its cross section is too small to encompassthe insulation, thereafter expanding the sheath and placing it about theinsulation and then allowing the sheath to contract about the insulationunder the influence of its own internal stresses.

In testimony whereof, we have hereunto set ARTHUR J. KERCHER. WILLIAMWESLEY HICKS.

